It is known to separate C.sub.4 hydrocarbons from a gaseous mixture containing the same, by cooling a feed gas partially to condense the same, and to separate organic and aqueous streams from the condensate. The organic fraction is removed from the cycle as product and the aqueous fraction is sewered.
The gaseous fraction is then dried and further cooled thereby partially to condense the same, the liquid phase being removed from the cycle as product and the gaseous phase being compressed to a pressure of about 300 psig.
The compressed gas is cooled in a main heat exchanger, in which it undergoes three stages of partial condensation. The liquid phase from each of the three separations is subjected to Joule-Thomson expansion and returned to the main exchanger to provide refrigeration. The liquid streams thus warmed in the main exchanger are removed from the cycle as product.
In the main exchanger, refrigeration is provided using successively and separately propane and then ethylene as the refrigerant liquids. Further refrigeration is provided by expanding isentropically the vapor remaining from the third operation of phase separation.
However, certain difficulties attend the known process. These are as follows:
1. Among the C.sub.4 hydrocarbons is 1,3-butadiene, which polymerizes at temperatures above 150.degree. F. The compression of the feed tends to raise the temperature of the feed above this temperature and bring about this polymerization.
2. The use of separate refrigerants at different temperature levels in the main heat exchanger results in discrete levels of refrigeration and leads to high power consumption.
3. The equipment for compressing the feed gas and for expanding it isentropically complicates the design and operation of the unit and increases its cost.